This is mainly a dynamic-range and uneven-illumination problem, so camera settings alone usually won’t fully fix it. The most practical approach is:

1. Keep the camera fixed on a tripod and the screen framed consistently.
2. Shoot RAW if possible, so you have more latitude for later adjustment.
3. Capture a “blank” frame of the reader with no fiche content visible. You can use that reference to subtract or normalize the hotspot across all images.
4. In post-processing, remove the brightness gradient with local normalization: create a heavily blurred/mean- or median-filtered version of the image and subtract it, then adjust contrast or threshold. This is well suited to automation or batch processing.
5. For manual correction, selective adjustments in Photoshop using masks can brighten the darker outer areas without blowing out the center.

If you need the best quality, avoid photographing the reader screen entirely: photograph the microfiche directly with a macro setup and even backlighting. The reader’s screen optics are part of the problem.

So: use a fixed setup, shoot RAW, and rely on gradient-removal/subtraction in post; for best results, digitize the fiche directly instead of the display.